RETICULAR THALAMIC REGION IN THE RABBIT - ORGANIZATION OF EFFERENTS TO THE SUPERIOR COLLICULUS

Although it is well-established that the reticular thalamic nucleus pr ovides a strong GABAergic input to the dorsal thalamus, the existence of reticular efferents to other subcortical centres is less certain. I n this study, we investigate whether the reticular nucleus projects to a major brainstem centre, the superior colliculus. The neuronal trace r, biotinylated dextran, was injected into superficial and deep layers of the superior colliculus of rabbits and the resultant labelling in the reticular region was examined. After large injections, which encom passed both superficial and deep collicular layers, two discrete popul ations of retrogradely labelled cells are seen in the region of the re ticular nucleus. One population of retrogradely labelled cells lies in the dorsocaudal regions of the reticular nucleus, the classically def ined visual sector. This group of retrogradely labelled reticular cell s is also seen after injections into the superficial layers of the sup erior colliculus, but not after injections limited to the deeper colli cular layers. The other population lies close to the ventromedial edge of the main body of the reticular nucleus, within a region referred t o as the inner small-celled region. This group of small cells has been commonly thought to be part of the reticular nucleus, but our immunoh istochemical studies suggest that this is a clearly separate region, a region continuous ventrally with zona incerta. The retrogradely label led cells in the inner small-celled region are seen also after injecti ons limited to the deeper collicular layers, but not after injections limited to the superficial collicular layers. Our results suggest func tional heterogeneity within the reticular nucleus: Specifically, it su ggests that the nucleus is in a position to influence the processing o f visual information at both the dorsal thalamic and midbrain levels. (C) 1996 Wiley-Liss, Inc.